Most jellyfish stings can be quickly treated with meat tenderizer, preferably mixed with vinegar. The juice of a green papaya (which can be used as a meat tenderizer) is also said to do the trick. If you don't have any of these items, you can even use your own urine, which is normally sterile. However, some jellyfish stings are not so harmless- for example a sting by the Portuguese man-o'war can be fatal.